The invention relates to a tensioner for an endless power transmission member such as an endless belt chain or the like, and a power transmission system that includes such a tensioner and power transmission member. More particularly, the invention relates to a spring type tensioner that biases the position of a pivot arm to which a rotatable pulley is mounted. While the tensioner of the invention may be used in various applications for tensioning and endless power transmission member, it is particularly useful in controlling belt tension of a V-ribbed belt as associated with front end accessory drives or a synchronous belt as associated with cam shaft drive systems for automotive applications.
Tensioners of the pivot arm type may use various types of spring bias means such as belville springs, volute springs, compression springs, or torsional coil springs. While the art discloses various types of spring means, the coil torsional spring is in prevalent use today in automotive applications. Examples of such tensioners with torsional springs are disclosed in U.S. Pat. No. 4,473,362 and U.S. Pat. No. 4,696,663. Although such tensioners are in wide spread use in automotive applications for satisfactorily controlling tension in a power transmission belt, the torsional springs used in such tensioners inherently introduce some design application problems which have to be considered for each power transmission system. For example, the torsional spring characteristically is positioned in some fashion about a pivot axis with one end operatively connected to a pivot arm and another end operatively connected to some type of support structure. Such an arrangement inherently introduces a couple about the pivot which must be carried both by the pivot and any pivot bushing that may be used.
Coiled torsional springs inherently introduce a variable force on the pivot arm which force varies in accordance with the spring's rate and consequently, the torque output of the pivot arm varies with the spring rate. Also in such tensioners, a component part of the spring force is typically used to effect variable damping of the pivot arm. In the '362 type of tensioner, a radially inward force results from winding the spring around the pivot and "pinching" an elastomeric bushing to effect a majority of a damping torque reacted at the pivot arm. the '663 tensioner, a radially outward force is generated by one of the spring ends which is used to press against a shoe type member against a cup to effect a damping torque reacted at the pulley. In both types of tensioners, the damping torque that is generated varies as a function of a spring rate and with movement of the pivot arm.
However, a goal in many tensioning applications is to provide a substantially constant force at a pulley so that a constant tension is achieved in the power transmission member such as an endless belt. To do this, torsional spring type tensioners must be mounted with their pivot arms angled in a geometric manner relative to the belt so that a trigonometric shortening and lengthening of the pivot arm compensates for variations introduced by the coiled torsional spring. Since the same spring is used to provide both a biasing force and a damping force in such tensioner designs, often times an iterative process is used to find a spring that will provide the necessary bias force and generate the necessary damping force for a particular geometric positioning of the pivot arm.
This invention is directed to circumvent the inherent drive design problems associated with coil torsional springs by using a compression spring to bias movement of a pivot arm and a second spring means to effect damping of the pivot arm. An example of a tensioner that uses a compression spring to bias movement of a pivot arm and a separate spring for damping appears in U.S. Pat. No. 4,299,584. In the '584 tensioner, the spring is retained in a tubular spring housing that is fixed to a support structure that does not move with movement of the pivot arm. The housing holds the spring in an aligned position relative to the support structure so that the compression spring like the torsional coil spring of the '362 and '663 patents, introduces a variable force on the pivot arm such that the torque output of the pivot arm varies with the spring rate. Also, the geometric arrangement of the '584 tensioner does not compensate for the variable torque output of the pivot arm and consequently, variable tension levels are introduced into the belt may be undesirable. Furthermore, the damping of the pivot arm is substantially constant because the damping means of a leaf spring exerting a constant force on a pad of friction material; this may be an undesirable combination with the variable torque output of the pivot arm for some drive applications.